Refactor parser AST into modules

[leynos]

Summary

• extract the parser::ast module into dedicated files
• keep small unit tests with clippy::expect_used allowances
• update parser::mod to reference the new module layout

Testing

• make lint
• make test

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https://chatgpt.com/codex/tasks/task_e_687b5d5a1ac88322be55977fc6787c38

Summary by Sourcery

Modularize the parser’s lightweight AST wrappers into dedicated files, introduce shared syntax utilities, and update module exports and tests accordingly.

New Features:

• Add syntax_utils::parse_parenthesized_list for extracting comma-separated lists from syntax elements
• Add parse_utils::extract_parenthesized helper for balanced delimiter extraction

Enhancements:

• Refactor parser::ast into separate modules for import, function, index, relation, rule, root, transformer, and type_def
• Update parser::mod and lib.rs to reference the new AST module structure and re-export utilities

Tests:

• Add focused unit tests for each AST wrapper and utility function with clippy::expect_used allowances